The present invention relates in particular to a process for producing a particulate composition comprising methionine, methionylmethionine, potassium salt and ammonium sulfate, and to the use thereof.
The amino acid methionine is currently industrially produced worldwide in large amounts and is of considerable commercial importance. Methionine is employed in many fields, such as pharmaceutical, health and fitness products, but particularly as feedstuff additive in many feedstuffs for various livestock. On an industrial scale, methionine is produced chemically via the Bucherer-Bergs reaction, which is a variant of the Strecker synthesis. Here, the starting substances 3-methylmercaptopropanal (prepared from 2-propenal and methylmercaptan), hydrocyanic acid (hydrogen cyanide), ammonia and carbon dioxide are reacted to give 5-(2-methylmercaptoethyl)hydantoin (methionine hydantoin) and this is subsequently hydrolysed by alkali with potassium carbonate and potassium hydrogen carbonate to give potassium methioninate. Methionine is finally liberated from its potassium salt by treatment with carbon dioxide, which may be filtered off as a precipitate from the mother liquor containing potassium carbonate and potassium hydrogen carbonate (U.S. Pat. No. 5,770,769). The ammonia, potassium carbonate and potassium hydrogen carbonate reagents and also carbon dioxide are generally recycled in industrial methionine production. From time to time, however, it is necessary to replace some of the aqueous mother liquor in this hydantoin hydrolysis circulation with fresh potassium hydroxide, essentially in order to remove the potassium salt, deactivated in the form of neutral potassium formate, from the circuit (“purge”). Potassium formate forms from the residues of hydrogen cyanide and potassium salts from the hydantoin hydrolysis present in the methionine hydantoin solution (WO2013030068A2). A further by-product of methionine synthesis is the dipeptide methionylmethionine (EP 1 564 208 A1). In general, the excessive enrichment of by-products in the hydantoin hydrolysis circulation must be avoided since otherwise disruptions in the crystal formation occur downstream.
The so-called purge solution comprises approximately 2 to 6% by weight methionine, 4 to 8% by weight methionylmethionine and 6 to 14% by weight potassium in the form of potassium salts. Due to the potassium, nitrogen and sulfur content, this solution is suitable as a liquid fertilizer (C. C. Mitchel and A. E. Hiltbold, Journal of Plant Nutrition, 17(12), 2119-2134, 1994). It was desirable, however, to provide such a fertilizer in solid form. Attempts to dewater this solution to form a solid, free-flowing solids mixture, in order to make it easier to store and transport this valuable material, have nevertheless hitherto failed (cf. Example 6 in this document).
The object on which the present invention is fundamentally based, accordingly, was that of providing a solid fertilizer based on methionine and potassium salts, and also a simple and cost-effective process for its production, in which in particular the aqueous mother liquor of the above-described hydantoin hydrolysis circulation can also be used as a material of value.